In radioactive waste disposal facilities, low-permeability engineered barriers are placed around radioactive waste to inhibit radionuclide migration. These barriers often comprise bentonite and cementitious materials; however, alteration of cementitious materials can produce alkaline leachates [1
]. Moreover, minerals in bentonite, such as montmorillonite, dissolve under alkaline conditions [2
], which could increase the permeability of bentonite barriers. On the other hand, the formation of secondary phases may decrease the permeability through a reduction of porosity [4
]. Therefore, it is important to analyze the dissolution of constituent minerals in bentonite, as well as the formation of secondary minerals, under alkaline conditions.
For the safety assessment of radioactive waste disposal, it is important to consider the long-term thermodynamics and/or kinetics of geochemical reactions in chemically complex systems. However, short-term laboratory experiments are typically conducted for relatively simple systems. For example, in order to determine a formula which describes the effects of pH on the dissolution kinetics of minerals, laboratory experiments should be conducted under different pH conditions using alkaline solutions such as NaOH or Ca(OH)2. The obtained formula can then be used to assess the lifetime of bentonite under alkaline disposal conditions. However, it is difficult to select a possible secondary mineral species because the composition of secondary mineral phases strongly depends on the experimental chemical system. Therefore, this study considers natural and long-term geochemical processes within a chemically complex system, particularly focused on the precipitation reactions under alkaline conditions.
Ultramafic rock-water interaction, which is known as present-day low-temperature serpentinization, can produce high-pH fluids (9–12) [5
]. Zeolite, K-feldspar, goethite, and Fe- and Mg-rich smectite were formed by the interaction between bentonite and alkaline water at the Saile bentonite-zeolite mine, close to the Zambales ophiolite in Luzon, the Philippines [7
]. Near the Troodos ophiolite in Cyprus, Fe-bearing palygorskite formation occurred through the interaction between bentonite and alkaline water over a time span of 105
]. Furthermore, precipitation of aragonite and magnesium silicate hydrates (M-S-H), which can be considered as poorly crystalline chrysotile, has been reported from the alkaline surface environment of an ultramafic body in the Sorachi ophiolite, Japan [9
]. Natural geochemical processes under alkaline conditions (pH 7–10) have also reported in an alkaline-saline lake [10
]. Alkaline-saline lakes of the Pantanal wetland in Brazil led to the formation of Fe-illite, glauconite, and Si-rich amorphous materials [10
]. In Searles Lake, California, where montmorillonite and illite are the principal detrital clay minerals, Fe-illite, Mg-smectite, K-feldspar, and zeolite were newly produced [11
]. Various Fe- and/or Mg-rich silicates (or silicate hydrates) do form under natural alkaline conditions; therefore, it is possible they could also form under alkaline conditions generated by the alteration of cementitious materials in the radioactive waste disposal facilities. However, it is still difficult to describe formation conditions of these minerals. Understanding the spatial distribution of these minerals, under natural alkaline conditions, may be crucial for understanding the conditions under which these minerals form in the radioactive waste disposal facilities.
At Narra, Palawan, the Philippines, alkaline fluids with a pH of over 11 have been naturally produced [12
]. A previous study has reported the formation of Fe, Mg, and Si-rich products as well as Ca and Si-rich products from the interaction between clastic sediments and alkaline fluids and discussed their potential formation mechanisms [12
]. However, the number of analyzed samples was limited, and the spatial distribution of the minerals remain uncertain. This study investigates the spatial distribution of primary and secondary minerals, as well as their depositional ages and depositional environment. The aim of this study is to understand the long-term dissolution–precipitation interactions and the conditions, which form different mineral species under alkaline conditions in a chemically complex system.
2. Site Description and Samples
The geological setting of the site in central Palawan is dominated by the Palawan ophiolite, which is composed of the Beaufort Ultramafic Complex, Stavely Gabbro, and Espina Formation [13
]. The Beaufort Ultramafic Complex comprises serpentinized peridotite and dunite, whereas Stavely Gabbro includes isotropic gabbro with minor layered gabbro [13
]. The main sampling site (Narra3-2; located at 9°12′14′′ N, 118°16′51′′ E, ~70 m above sea level) is an alluvial fan channel spreading on a gentle slope of the Palawan ophiolite basement (Figure 1
a,b). Eight trenches (T1–8) and four drill holes (DH1–4) were excavated at Narra3-2 (Figure 1
b). The basement was found at a depth of 11.5 m in DH4, which was the deepest drill hole. Above the basement, clastic sediments and covered carbonate layers are present. The thickness of the carbonate layer decreases from the fan apex side (>2.2 m) to the fan toe (<0.2 m at T6 and T7) (Figure 1
b and Figure 2
a,b). Alkaline spring water emanates along fractures at Narra3-1, upstream of the alluvial fan, i.e., Narra3-2 (Figure 1
b). At Narra3-2, alkaline water with similar chemical characteristics to the spring water of Narra3-1 seeps from the trench walls and drill holes after excavation, except at T8, which is located on the slope of a small hill. The alkaline seepage originates from the interaction of meteoric water with the ultramafic rocks [12
]. As this interaction occurred upstream of Narra3-2, alkaline water may have been seeping into Narra3-2 on a geological time scale. The flow velocity of the alkaline seepage was measured via simplified pumping test using the excavated cavity of T5, and the volume of the stored alkaline seepage over 0.75 days was approximately 17 m3
c). Assuming that the stored alkaline water was seeping from an 18-m2
front into the trench facing upstream, the estimated flow velocity was approximately 1.26 m3
/day. Although this estimate may contain errors and does not consider changes in flow velocity due to seasonal changes in the amount of rainwater or changes in clastic sediment permeability, advection of alkaline seepage could account for major long-term mass transfer.
Solid samples were collected from each trench and drill hole at depths from 0–16 m and numbered in order from bottom to top, for example, the deepest samples collected from T1 were labeled “T1-1.” The results of T5, T2, T4, T3, DH4, and DH1 are summarized in section A-a, which is oriented perpendicular to the flow direction of the alkaline seepage, whereas the results of T1, DH3, T2, T6, and T7 are summarized in section B-b, which extends along the fan (Figure 1
b). In the field, sections A-a and B-b intersect at right angles at T2; therefore, this trench occurs in both sections. T1 is located at the fan apex side, while the trenches and drill holes in section A-a and DH3 are located in the middle of the fan, and T6 and T7 are located at the fan toe. Solid samples were also collected from outcrops near Narra3-2 devoid of alkaline fluids (denoted as ST2), to compare their mineralogical composition to that of the sediments at Narra3-2 (Figure 1
a and Figure 2
d). Alkaline seepage was collected from T1–T7 and DH1–DH4 at a depth of 1–11 m at Narra3-2, and spring water was collected at Narra3-1. These waters generally have high Ca content (10.4–48.5 ppm), pH > 11, and temperatures of 27–36 °C [12
]. Although their Ca contents are lower than the Ca abundance of the pore fluids from low-alkaline cements (300–1200 ppm) [17
], the pH values of these waters are similar to those of pore fluids from low-alkaline cements (pH = 10–12) used in the radioactive waste disposal facilities.
The radioactive carbon (14
C) age and the stable carbon isotope ratio (13
C) of humin, an alkaline-insoluble organic matter, in the clastic sediments were measured. The 14
C content of humin decayed after the biological activity of original organic materials in humin ceased. Therefore, the differences of the 14
C age of humin in the clastic sediments could be affected by the differences of their depositional ages. On the other hand, the stable carbon isotope ratio of humin is reported with the conventional δ notation (δ13
C value) as the per mil (‰) deviation from the isotope ratio of the sample to that of the Pee Dee belemnite (PDB) carbonate standard. As a result that the mean δ13
C value of organic matter in marine muds is −20‰ and for freshwater sediments is −25‰ [18
], estimating the δ13
C value of humin is a useful index for the determination of the depositional environments of the related sediments. Preprocessing procedures and measurements were conducted by Beta Analytic Inc. (Miami, FL, USA), and the humin was extracted from each sample using an acid-alkali-acid (AAA) pre-treatment (see www.radiocarbon.com
). Measurements were conducted using accelerator mass spectrometry (AMS), and the isotopic distribution in sections A-a and B-b were illustrated as contour maps through Kriging interpolation within the convex hull for each data (Surfer 16, Golden Software (Golden, CO, USA)).
X-ray diffraction (XRD) analysis was conducted using pulverized and randomly oriented samples to determine their mineralogical compositions. The samples were examined using an X-ray diffractometer (SmartLab, Rigaku (Akishima-shi, Tokyo, Japan)) with CuKα radiation at 45 kV and 50 mA, at a scanning speed of 2 °/min and a scanning step of 0.02°. To estimate the mineral abundance of each sample, the intensity of the peaks assigned to each mineral were measured from the XRD profile after peak separation. Specifically, the intensity values of peaks with 7.3 Å d-spacing for serpentine, 8.4 Å d-spacing for amphibole, 3.0 Å d-spacing for calcite, and 14 Å d-spacing for tobermorite were measured. The peak intensity of orthopyroxene was the sum of the peak intensities for 3.2 Å and 2.9 Å. Moreover, the peak intensity of a broad peak with approximately 14 Å d-spacing was also measured after peak separation of tobermorite and chlorite due to the overlap of their peaks. Although it is difficult to accurately quantify mineral contents using this method, it is useful for determining the relative abundance of each mineral. The peak intensity data in sections A-a and B-b were then similarly converted into a contour map through Kriging interpolation within the convex hull for each data.
Thin sections of selected samples were observed, and microanalyses were acquired using a field emission scanning electron microscope with energy dispersive X-ray spectroscopy (FESEM-EDS; JSM-7001F, JEOL (Akishima-shi, Tokyo, Japan)). The medians of chemical compositions of intergranular infillings in each sample were calculated, and then the median values of each sample from sections A-a and B-b were similarly converted into a contour map through Kriging interpolation within convex hull for each data. Some of the data were taken from a previous report [19
To obtain the <2-μm fraction from bulk samples, the samples were dispersed in distilled water and separated by centrifugation. Oriented samples were prepared from this <2-μm fraction with the suspension of 10 mg of dried material in distilled water and sedimentation onto a glass slide. XRD analysis of the oriented samples was conducted using an X-ray diffractometer (SmartLab, Rigaku (Akishima-shi, Tokyo, Japan)) with CuKα radiation at 45 kV and 30 mA, at a scanning speed of 5 °/min, and a scanning step of 0.01°. After analysis, the oriented samples were solvated with ethylene glycol (EG) vapor at 60 °C for more than 12 h. XRD analysis of the EG-solvated samples was conducted using the same conditions to identify the expandable features of mineral containing a broad peak with 14 Å d-spacing.
The randomly oriented samples of the <2-μm fraction were further measured by XRD analysis to observe the 060 reflection, which is routinely used to estimate the degree of occupancy in the octahedral sheet of clay minerals. XRD analysis was similarly conducted with CuKα radiation at 45 kV and 30 mA at a scanning speed of 0.3 °/min and a scanning step of 0.01°. XRD analysis of the randomly oriented samples for the <2-μm fraction during heating at 250 °C was performed to determine changes in the peak position in the dehydrated state. XRD patterns were collected after maintaining the temperature for 2 h. XRD analysis was conducted using the same instrument and conditions as above except a scanning speed of 5 °/min and a scanning step of 0.02°.
The Geochemist’s Workbench (GWB; Aqueous Solutions (Champaign, IL, USA)) software package was used for thermodynamic calculations. The calculations employed the thermodynamic database, “Thermoddem_V1.06d” (2009), provided by the Bureau de Recherches Géologiques et Minieres (BRGM). The saturation index of alkaline water with respect to each mineral was calculated. The chemical compositions of alkaline waters collected from Narra3-1, T1–T7, DH1, DH3, and DH4 were obtained from previous studies [12
]. The charge in alkaline water was balanced by Cl−
ions, and dissolved ion concentrations below 0.01 ppm were assumed to be 0.01 ppm. Moreover, the calculation also assumed that the dissolved iron, which is the sum of the dissolved Fe2+
ions, is ferrous and dissolved Fe2+
ions were used to calculate the saturation index with respect to Fe-bearing minerals. These assumptions may have led to slightly high calculated saturation indices with respect to each mineral. The geochemical changes induced by mixing seawater or freshwater with alkaline seepage helped to predict the changes of the fluid chemistry and minerals formation. Our calculations assumed that the minerals form instantaneously in equilibrium with the mixed solution. The chemical compositions of alkaline waters collected from Narra3-1 were considered representative of alkaline seepage in this study, and the charge in alkaline water was balanced by Cl−
ions. Finally, the chemical compositions of general seawater and freshwater were obtained from a previous study [20
This study reveals the temporal-spatial distribution of primary and secondary minerals and their related depositional environments to understand long-term dissolution–precipitation reactions under alkaline conditions in a chemically complex system. The deposition of clastic sediments at Narra, derived from serpentinized ultramafic rocks and gabbro of the Palawan ophiolite, started 15,000 yr BP, as it has been documented by 14C dates. The depositional environment gradually changed from seawater to brackish water at the fan toe and finally to freshwater in the middle of the fan. The clastic sediments interacted with alkaline seepage with a pH of over 11, forming two main phases: (1) Fe-Mg-Si infillings, and (2) Ca-Si infillings. The Fe-Mg-Si infillings were observed widely in the clastic sediments and differed with increasing depth from F-M-S-H to a nontronite-like mineral in the middle of the fan. In contrast, the nontronite-like mineral formed even in the shallower parts of the fan toe. The distribution of the nontronite-like mineral suggests that infiltration of seawater during the interaction between clastic sediments and alkaline seepage could be responsible for its formation. Moreover, the nontronite-like mineral was stable under present-day alkaline conditions after the end of the seawater infiltration, for a period of thousands of years. Conversely, F-M-S-H was formed by the interaction between clastic sediments and alkaline seepage without the involvement of seawater. The Ca-Si infillings include 14 Å tobermorite, which formed at depths of 4 m just below the calcite-rich surface layer. The precipitation of tobermorite was related to the dissolution of calcite and/or silicates in the clastic sediments due to interaction with alkaline seepage. This study reveals long-term dissolution–precipitation interactions and the conditions that form different mineral species under alkaline conditions. This may help to select suitable mineral species precipitated under similar alkaline conditions which were generated by the alteration of low-alkaline cement within radioactive waste disposal facilities.